Method of scaling navigation signals to account for impedance drift in tissue

ABSTRACT

A method for scaling the impedance measured during the course of an electrophysiology study accounts for impedance drifts. By scaling the impedance there is greater assurance that previously recorded positional information can be used to accurately relocate an electrode at a prior visited position. The scale factor may be based upon a mean value across several sensing electrodes. Alternatively, the scale factor may be calculated specifically with respect to an orientation of a dipole pair of driven electrodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to systems for positioning and mappingelectrophysiology catheters and ablation catheters in the heart of apatient. The invention further relates to methods for error correctionin electrocardiograph signals.

2. Background Art

U.S. Pat. Nos. 5,697,377 (the '377 patent) and 5,983,126 (the '126patent) to Wittkampf disclose a system for determining the position orlocation of a catheter in the heart. The '377 patent and the '126 patentare hereby incorporated herein by reference in their entirety. In theWittkampf system, current pulses are applied to orthogonally placedpatch electrodes placed on the surface of the patient. These surfaceelectrodes are used to create axis specific electric fields within thepatient. The Wittkampf references teach the delivery of small amplitude,low current pulses supplied continuously at three different frequencies,one on each axis. Any measurement electrode placed in these electricfields (for example within the heart) measures a voltage that variesdepending on the location of the measurement electrode between thevarious surface electrodes on each axis. The voltage across themeasurement electrode in the electric field in reference to a stablepositional reference electrode indicates the position of the measurementelectrode in the heart with respect to that reference. Measurement ofthe difference in voltage over the three separate axes gives rise topositional information for the measurement electrode in threedimensions.

Although the Wittkampf system is both safe and effective there areseveral factors that can result in errors in the position of themeasurement electrode. Some factors previously identified as sources ofimpedance modulation include the cardiac cycle and respiration. Both ofthese sources also cause actual physical movement of an electrode inaddition to direct impedance effects. Mitigations to these modulators toenhance stability of electrode positional measurements include low passfiltering, cardiac cycle triggering, and respiration compensation. Onefactor not previously addressed is the tendency of biologic impedance tochange over time. Changes in biologic impedance are attributable tochanges in cell chemistry, for example, due to saline or other hydrationdrips in the patient, dehydration, or changes in body temperature.

If the biologic impedance changes over a longer term (i.e., minutes orhours), then apparent shifts of the measured locations of electrodes mayoccur. If an internal cardiac electrode is used as a referenceelectrode, these shifts may be negligible, since they are manifest as ascale factor change of only a few percent. For example, a 2 percentchange with respect to a fixed reference 4.0 centimeters away willrepresent an error of 0.8 millimeters, which is generally consideredacceptable. However, if it is desired to use an external body surfaceelectrode as a fixed reference, and eliminate the requirement of a fixedintra-cardiac electrode reference, then 2 percent may represent anintolerable error source. For example, if the reference electrode is an“apparent” 40 cm from a mapping electrode, the error due to a 2%impedance drift would be 8 millimeters. The term “apparent” is usedbecause while the actual distance to the reference electrode may besomewhat less, the intervening biologic of lung and muscle tissue ishigher than that of blood, such that it scales to a larger distance.

The information included in this Background section of thespecification, including any references cited herein and any descriptionor discussion thereof, is included for technical reference purposes onlyand is not to be regarded subject matter by which the scope of theinvention is to be bound.

SUMMARY OF THE INVENTION

The present invention addresses the problem of biologic impedancechanges and the effect on measurement of the position of an electrodewithin a patient by continuously calculating a scale factor to apply toimpedance measurements throughout the course of a procedure. Changes inbiologic impedance attributable to changes in cell chemistry, forexample, due to saline or other hydration drips in the patient,dehydration, or changes in body temperature, can thus be accounted forand a more accurate positional reading for a measurement electrode maybe obtained.

In one form, the invention may be understood as a method for scalingimpedance measurements in an electrophysiology study. A first dipole isdriven along a first axis to create an electric field across a patient'sbody. A biologic impedance encountered by the electric field withrespect to a surface sensor is measured. The mean of the absolute valueof the measured biologic impedance is calculated continuously as afunction of time Pm(t). An initial calculated mean of the absolute valueof the at least one measured biologic impedance is saved as Pa. Animpedance measurement between a measurement electrode and a referenceelectrode is then multiplied by the ratio of Pa/Pm(t) to scale theimpedance measurement and account for any drift. This scalingcalculation may be performed by software controlling anelectrophysiology study or ablation system. The invention may furtherbreak the scale factors into component scales with respect to aplurality of dipole axes that may be driven. In this instance, separatemeasurements for each axis are measured and the scale factor isrepresented by Pa(i)/Pm(t,i), where (i) indicates the axis ofmeasurement.

Other features, details, utilities, and advantages of the presentinvention will be apparent from the following more particular writtendescription of various embodiments of the invention as furtherillustrated in the accompanying drawings and defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for performing a cardiacelectrophysiology examination or ablation procedure wherein the locationof one or more electrodes can be determined and recorded.

FIGS. 2A-2D are schematic diagrams of dipole pairs of driven surfaceelectrodes.

FIG. 3 is a table indicating the surface electrodes used as sensors whena particular dipole pair of surface electrodes is driven.

FIG. 4 is a schematic diagram of biologic impedance drift portrayed asan electrical circuit.

DETAILED DESCRIPTION OF THE INVENTION

One of the primary goals of cardiac electrophysiology mapping is tolocate with some certainty the position of an electrode within a cardiaccavity. An electric navigation field is created within a patient's bodyon each of the three principal axes by driving a constant current. Ifthe impedances measured in the body are constant, then the potentials oneach axis measured at a location, with respect to a reference electrodeat a static location, will remain at a constant potential over time.Thus, if a location site is marked in the heart, one may return to thatsite with a catheter electrode in the future and be confident that ifthe measured navigation potentials, or impedance, are the same asbefore, the anatomic location is the same.

FIG. 1 depicts a schematic diagram of an exemplary electrophysiologymapping or ablation system. The patient 11 is depicted as an oval forclarity. Three sets of surface electrodes (e.g., patch electrodes) areshown applied to a surface of the patient 11 along an X-axis, a Y-axis,and a Z-axis. The X-axis surface electrodes 12, 14 are applied to thepatient along a first axis, such as on the lateral sides of the thoraxregion of the patient (e.g., applied to the patient's skin underneatheach arm) and may be referred to as the Left and Right electrodes. TheY-axis electrodes 18, 19 are applied to the patient along a second axisgenerally orthogonal to the X-axis, such as along the sternum and spineof the patient in the thorax region and may be referred to as the Chestand Back electrodes. The Z-axis electrodes 16, 22 are applied along athird axis generally orthogonal to both the X-axis and the Y-axis, suchas along the:inner thigh and neck regions of the patient, and may bereferred to as the Left Leg and Neck electrodes. The heart 10 liesbetween these pairs of surface electrodes. An additional surfacereference electrode (e.g., a “belly patch”) 21 provides a referenceand/or ground electrode for the system 8. The belly patch electrode 21is an alternative to a fixed intra-cardiac electrode 31. It should alsobe appreciated that in addition, the patient 11 will have most or all ofthe conventional electrocardiogram (ECG) system leads in place. This ECGinformation is available to the system although not illustrated in theFIG. 1.

A representative catheter 13 with a single, distal measurement electrode17 is also depicted in FIG. 1. The catheter 13 may also have additionalelectrodes in addition to the measurement electrode 17. A fixedreference electrode 31 may be attached to a heart wall on an independentcatheter 29. In many instances, a coronary sinus electrode or otherfixed reference electrode 31 in the heart 10 can be used as a referencefor measuring voltages and displacements. For calibration purposes thereference electrode 31 remains stationary on the wall of the heartduring the course of the procedure.

Each surface electrode is independently connected to a multiplex switch24. Pairs of the surface electrodes are selected by software running ona computer 20, which couples the surface electrodes 12, 14, 16, 18, 19,21, 22 to a signal generator 25. A first pair of surface electrodes, forexample, the Z-axis electrodes 18, 19, is excited by the signalgenerator 25. The exited electrodes generate an electric field in thebody of the patient 11 and the heart 10. This electrode excitationprocess occurs rapidly and sequentially as alternate sets of patchelectrodes are selected and one or more of the unexcited surfaceelectrodes are used to measure voltages. During the delivery of acurrent pulse, the unexcited surface electrodes 12, 14, 16, and 22 arereferenced to either the reference electrode 31 or the belly patch 21and respective voltages are measured across one or more of theseunexcited electrodes. In this way, the surface electrodes are dividedinto driven and non-driven electrode sets.

While a pair of electrodes is driven by the current generator 25, theremaining, non-driven electrodes may be used as references to synthesizethe orthogonal drive axes. A low pass filter 27 processes the voltagemeasurements to remove electronic noise and cardiac motion artifact fromthe measurement signals. The filtered voltage measurements aretransformed to digital data by the analog to digital converter 26 andtransmitted to the computer 20 for storage under the direction ofsoftware. This collection of voltage measurements is referred to hereinas the “patch data.” The software has access to each individual voltagemeasurement made at each surface electrode during each excitation ofeach pair of surface electrodes. The patch data is used to determine arelative location in three dimensions (X, Y, Z) of the measurementelectrode 17. Potentials across each of the six orthogonal surfaceelectrodes may be acquired for all samples except when a particularsurface electrode pair is driven. Sampling while a surface electrodeacts as a source or sink in a driven pair is normally avoided as thepotential measured at a driven electrode during this time will be skewedby the electrode impedance and the effects of high local currentdensity.

Generally, three nominally orthogonal electric fields are generated by aseries of driven and sensed electric dipoles in order to realizecatheter navigation in a biological conductor. Alternately, theseorthogonal fields can be decomposed and any pairs of surface electrodescan be driven as dipoles to provide effective electrode triangulation.Additionally, such nonorthogonal methodologies add to the flexibility ofthe system and the ability to localize biologic impedance compensation.For any desired axis, the potentials measured across an intra-cardiacelectrode 17 resulting from a predetermined set of drive (source-sink)configurations are combined algebraically to yield the same effectivepotential as would be obtained by simply driving a uniform current alongthe orthogonal axes.

Thus, any two of the surface electrodes 12, 14, 16, 18, 19, 22 may beselected as a dipole source and drain with respect to a groundreference, e.g., the belly patch 21, while the unexcited electrodesmeasure voltage with respect to the ground reference. The measurementelectrode 17 placed in the heart 10 is exposed to the field from acurrent pulse and is measured with respect to ground, e.g., the bellypatch 21. In practice the catheters within the heart may containmultiple electrodes and each electrode potential may be measured. Aspreviously noted, at least one electrode may be fixed to the interiorsurface of the heart to form a fixed reference electrode 31, which isalso measured with respect to ground. Data sets from each of the surfaceelectrodes and the internal electrodes are all used to determine thelocation of the measurement electrode 17 within the heart 10. After thevoltage measurements are made, a different pair of surface electrodes isexcited by the current source and the voltage measurement process of theremaining patch electrodes and internal electrodes takes place. Thesequence occurs rapidly on the order of 100 times per second. To a firstapproximation the voltage on the electrodes within the heart bears alinear relationship with position between the patch electrodes thatestablish the field within the heart. Correction factors, e.g., tocompensate for respiration, may be applied to the raw locationinformation to improve the accuracy of the location value.

When operating with constant currents during an electrophysiology study,the potentials created with respect to any reference will be a functionof the intervening impedance. This concept is represented schematicallyin FIG. 4. A catheter 13 with a distal measurement electrode 17 isplaced within the heart 10 of the patient 11. A set of orthogonalsurface electrodes 12, 14, 16, 18, 19, 22 are alternately paired asdrive electrodes while the remaining, unexcited electrodes may functionas sensor electrodes. The electric potential generated by a dipole pairof electrodes may be measured at a measurement electrode in the body andthe path of current through the body between the driven surfaceelectrode and the distal measurement electrode 17 on the catheter may beunderstood simply as a circuit. For example, as shown in FIG. 4, whenthe Left Leg electrode 18 is driven as a source, a circuit 30 is createdbetween the Left Leg electrode 18 and the measurement electrode 17 onthe catheter 13 within the heart 10. The body tissue between the skin atthe Left Leg electrode 12 and the heart 10 acts primarily as a resistiveimpedance and can be viewed functionally as a resistor 32. However, dueto changes in body chemistry during a procedure, the value of thisresistor 32 may drift. Therefore, the resistor 32 may be more accuratelyviewed as a variable resistor as depicted. It should be apparent that ifthe tissue impedance changes, then the voltage data and thus positionaldata for each electrode, including a reference electrode, e.g., thebelly patch electrode 21, will similarly drift.

To the extent that intervening impedances change after a study beginsand sites have been marked, attempts to place the measurement electrodeat previously visited locations may not be consistent with earliermarked sites. To the extent that the impedance change for a given pairof drive surface electrode dipoles is tracked by unexcited surfaceelectrodes used as sensors, these data may be used to correct the drifton each driven dipole. This is best done by using the surface electrodepatches as sensors in the time slices when they are un-driven. Twoexemplary methods are discussed herein to account for and correct anypotential drift. The first method assumes that the bio-impedance changesare essentially homogeneous. Thus all driven dipoles change by the samepercentage and averaging of all the patch data returns a singleimpedance index. The second method does not make this assumption andcomputes an index for each axis.

A first method for biologic impedance scaling assumes that any biologicimpedance changes are essentially homogenous within the body, that alldipoles change by the same percentage, and that averaging of all surfaceelectrode data will provide a single, accurate impedance index. Themethod uses the Neck, Left, Right, Chest, and Back surface electrodepatches as sensors when they are not being driven. The Left Legelectrode is not generally used for sensing because sensed potentials onit tend to be very small. For pairings of the driven surface electrodeexcluding the Left Leg, there are thus three potential sense patchesthat contribute data. In the case of driven pairings including the LeftLeg, four surface electrode patches are available as sensors. The meanof the absolute values of the measured impedance at the surfaceelectrode sensors is obtained continuously as a function of time. Thismean value may be denoted as Pm(t). Immediately after the a studybegins, the initial value of Pm(t) is saved as an initial value Pa. Onevery subsequent sample, all of the measurement electrode data ismultiplied by the ratio Pa/Pm(t). The three dimensional impedancemeasurements are thereby scaled to account for any drift over time.

To illustrate, a study begins and, assuming there are no errors due tounconnected surface electrodes, the surface electrodes are each sensedduring their un-driven phases and averaged to yield a Pa of 10.0 ohms,which is saved in the software on the computer 20. Assume an importantsite is marked with a measurement electrode on a catheter in the heartat a site with the impedance coordinates of (1.0, 2.0, 10.0) ohms withrespect to a belly patch reference electrode 21. These impedancecoordinates are translated for the user using a nominal scale factor toyield positional coordinates of (25, 50, 400) millimeters. Suppose, dueto a saline drip or other factors, an hour later the patient's biologicimpedance has lowered by 2%. Without compensation, a return visit by acatheter electrode to the marked anatomic site would now show up atcoordinates of (24.5, 49.0, 392.0) millimeters, a drift of 8millimeters. However, the same biologic impedance drift will beregistered on the data sensed by the surface electrodes. For example,Pm(t) will now read 9.8 ohms. Therefore, scaling the catheter electrodecoordinate data by Pa/Pm(t), or 1.0204, will ensure the catheterelectrode is positioned at its original and correct location.

A second method for scaling biologic impedance shifts according to thepresent invention is generally the same as in the prior embodiment,except that separate scale factors are maintained for each axis or pairof driven electrode dipoles. While it is generally found that biologicimpedance drifts are essentially homogeneous, this method may bepreferred in the event that biologic impedance drifts are nothomogeneous throughout the measurement space. In this instance a value,Pm(i,t), the mean of the absolute values of the measured impedance ofeach of the dipole combinations as a function of time, can becalculated. In this method, Pm(i,t) for each dipole pair are computed,where “i” is the dipole number, or “Impedance Number,” by averaging theimpedance measurements at the surface electrodes on a dipole basis.Notably, not all of the undriven surface electrodes need be averaged tooptimally determine the axial impedance for each dipole. Thus, theinitial value of patch impedance, Pa(i), and subsequent values of patchimpedance, Pa(i,t), may be measured for each dipole by averaging onlythe electrode data per the table of FIG. 3 for that dipole.

As shown in FIG. 3, the measured impedance for a first dipole Xa-Ya (0)between the Left electrode and the Back electrode may be obtained byaveraging the Right electrode (Xb) and the Neck electrode (Za) data.This dipole is depicted in FIG. 2A. Similarly, the measured impedancefor a second dipole Xa-Yb (1) between the Left electrode and the Chestelectrode may also be obtained by averaging the Right electrode (Xb) andthe Neck electrode (Za) data. This dipole is depicted in FIG. 2B. Themeasured impedance for a third dipole Xb-Ya (2) driven between the Rightelectrode and the Back electrode may be obtained by averaging the Leftelectrode (Xb) and the Chest electrode (Yb) data. This dipole isdepicted in FIG. 2C. The measured impedance for a fourth dipole Xb-Yb(3) driven between the Right electrode and the Chest electrode may beobtained by averaging the Left electrode (Xb), the Back electrode (Ya)and the Neck electrode (Yb) data together. This dipole is depicted inFIG. 2D. The measured impedance for a fifth dipole Za-Ya (4) drivenbetween the Neck electrode and the Back electrode may be obtained byaveraging the Left electrode (Xa) and the Right electrode (Xb) datatogether. Finally, the measured impedance for a sixth dipole is Zb-Ya(5) driven between the left Leg electrode and the Back electrode may beoptimally obtained by recoding date for the Chest electrode (Xa) only.

It should be noted that while biologic impedance scaling will have thegreatest effect when correcting impedance changes when a body surfaceelectrode is used as a reference electrode, such scaling may also beapplied to cases where an intra-cardiac reference electrode is used.There is simply less error to correct for in the latter case.

Although various embodiments of this invention have been described abovewith a certain degree of particularity, or with reference to one or moreindividual embodiments, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thespirit or scope of this invention. All directional references (e.g.,proximal, distal, upper, lower, upward, downward, left, right, lateral,front, back, top, bottom, above, below, vertical, horizontal, clockwise,and counterclockwise) are only used for identification purposes to aidthe reader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Connection references (e.g., attached, coupled, connected,and joined) are to be construed broadly and may include intermediatemembers between a collection of elements and relative movement betweenelements unless otherwise indicated. As such, connection references donot necessarily infer that two elements are directly connected and infixed relation to each other. It is intended that all matter containedin the above description or shown in the accompanying drawings shall beinterpreted as illustrative only and not limiting. Changes in detail orstructure may be made without departing from the basic elements of theinvention as defined in the following claims.

1. A method for scaling impedance measurements in an electrophysiologystudy, the method comprising driving a first dipole along a first axisto create an electric field across a patient's body; measuring at leastone biologic impedance encountered by the electric field with respect toat least one surface sensor; calculating the mean of the absolute valueof the at least one measured biologic impedance continuously as afunction of time Pm(t); saving an initial calculated mean of theabsolute value of the at least one measured biologic impedance as Pa;and multiplying an impedance measurement between a measurement electrodeand a reference electrode by the ratio of Pa/Pm(t).
 2. The method ofclaim 1, wherein the at least one surface sensor comprises a pluralityof surface sensors; the step of calculating further comprisescalculating the mean of the absolute values of the measured biologicimpedance as a function of time at each of the plurality of surfacesensors; and the step of saving further comprises saving the initialcalculated mean of the absolute values of the of the measured biologicimpedance at each of the plurality of surface sensors.
 3. The method ofclaim 1, wherein the step of driving further comprises driving a seconddipole along a second axis across the patient's body; the step ofcalculating further comprises calculating means of the absolute valuesof the measured biologic impedance as a function of time at the at leastone surface sensor with respect to both the first dipole and the seconddipole separately as Pm(t,i); the step of saving further comprisessaving an initial calculated mean of the absolute values of the measuredbiologic impedance for each of the first dipole and the second dipoleseparately as Pa(i); and the step of multiplying further comprisesmultiplying an impedance measurement at the measurement electrode withrespect to each of the first dipole and the second dipole by therespective ratio of Pa(i)/Pm(t,i).
 4. The method of claim 3, wherein theat least one surface sensor comprises a plurality of surface sensors;the step of calculating further comprises calculating the mean of theabsolute values of the measured biologic impedance as a function of timeat each of the plurality of surface sensors; and the step of savingfurther comprises saving the initial calculated mean of the absolutevalues of the of the measured biologic impedance at each of theplurality of surface sensors.
 5. The method of claim 3, wherein thefirst axis and the second axis are generally orthogonal to each other.6. The method of claim 3, wherein the step of driving further comprisesdriving a third dipole along a third axis across the patient's body; thestep of calculating further comprises calculating means of the absolutevalues of the measured biologic impedance as a function of time at theat least one surface sensor with respect to each of the first dipole,the second dipole, and the third dipole separately as Pm(t,i); the stepof saving further comprises saving an initial calculated mean of theabsolute values of the measured biologic impedance for each of the firstdipole, the second dipole, and the third dipole separately as Pa(i); andthe step of multiplying further comprises multiplying an impedancemeasurement at the measurement electrode with respect to each of thefirst dipole, the second dipole, and the third dipole by the respectiveratio of Pa(i)/Pm(t,i).
 7. The method of claim 6, wherein the at leastone surface sensor comprises a plurality of surface sensors; the step ofcalculating further comprises calculating the mean of the absolutevalues of the measured biologic impedance as a function of time at eachof the plurality of surface sensors; and the step of saving furthercomprises saving the initial calculated mean of the absolute values ofthe of the measured biologic impedance at each of the plurality ofsurface sensors.
 8. The method of claim 6, wherein the first axis, thesecond axis, and the third axis are generally orthogonal to each other.9. The method of claim 1, wherein the step of driving compriseselectrically exciting a pair of surface electrodes on the patient's bodyto act as a source and a drain, respectively.
 10. A method fordetermining a position of at least one measurement electrode within apatient's body, the method comprising positioning a plurality of surfaceelectrodes on a surface of the patient's body; electrically driving afirst pair of the plurality of surface electrodes on the patient's bodyto act as a source and a drain; sensing at least one impedance value atat least one of the plurality of surface electrodes that is not one ofthe driven pair of surface electrodes; measuring an impedance betweenthe at least one measurement electrode and a reference electrode;repeating the steps of driving, sensing, and measuring for a second pairof the surface electrodes and a third pair of the surface electrodes;scaling the impedance sensed at at least one of the surface electrodesfor each step of sensing and measuring by calculating the mean of theabsolute value of the at least sensed impedance continuously as afunction of time Pm(t); saving an initial calculated mean of theabsolute value of the at least one sensed impedance as Pa; andmultiplying the impedance measurement between the at least onemeasurement electrode and the reference electrode by the ratio ofPa/Pm(t); and identifying the position of the at least one measurementelectrode within the patient's body as a function of the scaledimpedance.
 11. The method of claim 10, wherein the step of calculatingfurther comprises calculating means of the absolute values of themeasured biologic impedance as a function of time at the at least onesurface sensor with respect to each of the first pair of the surfaceelectrodes, the second pair of the surface electrodes, and the thirdpair of the surface electrodes separately as Pm(t,i); the step of savingfurther comprises saving an initial calculated mean of the absolutevalues of the measured biologic impedance for each of separately asPa(i); and the step of multiplying further comprises multiplying animpedance measurement at the measurement electrode with respect to eachof the first pair of the surface electrodes, the second pair of thesurface electrodes, and the third pair of the surface electrodes by therespective ratio of Pa(i)/Pm(t,i).
 12. The method of claim 10, whereinrespective axes between each of the first, second, and third pairs ofsurface electrodes are generally orthogonal to each other.
 13. Themethod of claim 10, wherein the step of sensing further comprisessensing the impedance value at each of the plurality of electrodes thatis not one of the driven pair of electrodes.
 14. The method of claim 10,wherein the step of sensing further comprises sensing the impedancevalue at a subset of the plurality of electrodes that does not includeone of the driven pair of electrodes.
 15. The method of claim 10,wherein the reference electrode is within the patient's body.
 16. Themethod of claim 10, wherein the reference electrode is on the surface ofthe patient's body.
 17. The method of claim 10, wherein the step ofpositioning further comprises categorizing the plurality of electrodesinto two or three pairs of electrodes orienting each pair of electrodessuch that an axis between electrodes in each pair of electrodes isgenerally orthogonal to axes between electrodes in the other pairs ofelectrodes.
 18. A computer-readable medium having computer-executableinstructions for performing steps comprising saving at least onebiologic impedance measurement encountered by at least one sensingelectrode on the surface of a patient's body under the influence of adipole driven electric field; calculating the mean of the absolute valueof the at least one measured biologic impedance continuously as afunction of time Pm(t); saving an initial calculated mean of theabsolute value of the at least one measured biologic impedance as Pa;and multiplying an impedance measurement between a measurement electrodeand a reference electrode by the ratio of Pa/Pm(t).
 19. The computerreadable medium of claim 18, wherein the step of calculating furthercomprises calculating means of the absolute values of the measuredbiologic impedance as a function of time at the at least one surfacesensor with respect to a plurality of dipole driven electric fieldsseparately as Pm(t,i); the step of saving further comprises saving aninitial calculated mean of the absolute values of the measured biologicimpedance for each of the plurality of dipole driven electric fieldsseparately as Pa(i); and the step of multiplying further comprisesmultiplying an impedance measurement at the measurement electrode withrespect to each of the plurality of dipole driven electric fields by therespective ratio of Pa(i)/Pm(t,i).